Matisse's painting "Reading Girl in White and Yellow" was stolen from Kunsthal museum in Rotterdam, Netherlands.

Photo: Peter Dejong, Associated Press

Matisse's painting "Reading Girl in White and Yellow" was stolen...

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Matisse's painting "Reading Girl in White and Yellow" was stolen from Kunsthal museum in Rotterdam, Netherlands.

Photo: Associated Press

Matisse's painting "Reading Girl in White and Yellow" was stolen...

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This photo released by the police in Rotterdam, Netherlands, on Tuesday, Oct. 16, 2012, shows the 1890 painting 'Self-Portrait' by Meyer de Haan. Dutch police say seven paintings stolen from the Kunsthal museum in Rotterdam include one by Pablo Picasso, one by Henri Matisse, and two by Claude Monet. The heist, one of the largest in years in the Netherlands, occurred while the private Triton Foundation collection was being exhibited publicly as a group for the first time. (AP Photo/Police Rotterdam)

Photo: Associated Press

This photo released by the police in Rotterdam, Netherlands, on...

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Forensic experts dust the inside of a back door for fingerprints at Kunsthal museum in Rotterdam, Netherlands, Tuesday Oct. 16, 2012. Seven paintings by artists including Pablo Picasso and Claude Monet were stolen from a museum in Rotterdam in an early-hours heist, Dutch police said Tuesday. The theft at the Kunsthal museum is one of the largest in years in the Netherlands, and is a stunning blow for the private Triton Foundation collection, which was being exhibited publicly as a group for the first time. (AP Photo/Peter Dejong)

Photo: Peter Dejong, Associated Press

Forensic experts dust the inside of a back door for fingerprints at...

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This photo released by the police in Rotterdam, Netherlands, on Tuesday, Oct. 16, 2012, shows the 1971 painting 'Harlequin Head' by Pablo Picasso. Dutch police say seven paintings stolen from the Kunsthal museum in Rotterdam include one by Pablo Picasso, one by Henri Matisse, and two by Claude Monet. The heist, one of the largest in years in the Netherlands, occurred while the private Triton Foundation collection was being exhibited publicly as a group for the first time. (AP Photo/Police Rotterdam)

Photo: Associated Press

This photo released by the police in Rotterdam, Netherlands, on...

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This photo released by the police in Rotterdam, Netherlands, on Tuesday, Oct. 16, 2012, shows the painting 'Waterloo Bridge, London' by Claude Monet. Dutch police say seven paintings stolen from the Kunsthal museum in Rotterdam include one by Pablo Picasso, one by Henri Matisse, and two by Claude Monet. The heist, one of the largest in years in the Netherlands, occurred while the private Triton Foundation collection was being exhibited publicly as a group for the first time. (AP Photo/Police Rotterdam)

Photo: Associated Press

This photo released by the police in Rotterdam, Netherlands, on...

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This photo released by the police in Rotterdam, Netherlands, on Tuesday, Oct. 16, 2012, shows the 1898 painting 'Girl in Front of Open Window' by Paul Gaugain. Dutch police say seven paintings stolen from the Kunsthal museum in Rotterdam include one by Pablo Picasso, one by Henri Matisse, and two by Claude Monet. The heist, one of the largest in years in the Netherlands, occurred while the private Triton Foundation collection was being exhibited publicly as a group for the first time. (AP Photo/Police Rotterdam)

Photo: Associated Press

This photo released by the police in Rotterdam, Netherlands, on...

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This photo released by the police in Rotterdam, Netherlands, on Tuesday, Oct. 16, 2012, shows the 1901 painting 'Charing Cross Bridge, London' by Claude Monet. Dutch police say seven paintings stolen from the Kunsthal museum in Rotterdam include one by Pablo Picasso, one by Henri Matisse, and two by Claude Monet. The heist, one of the largest in years in the Netherlands, occurred while the private Triton Foundation collection was being exhibited publicly as a group for the first time. (AP Photo/Police Rotterdam)

The heist at the Kunsthal museum is one of the largest in years in the Netherlands, and is a stunning blow for the private Triton Foundation collection, which was being exhibited publicly as a group for the first time.

"It's every museum director's worst nightmare," said Kunsthal director Emily Ansenk, who had been in Istanbul on business but returned immediately.

She declined to reveal details of how the thieves got in and out with the paintings, or how the museum is protected, other than describing its security as "state of the art" and "functional."

Willem van Hassel, the museum's chairman, said its security systems are automated, and do not use guards on site.

Police spokeswoman Willemieke Romijn said investigators were reviewing videotapes of the theft, which took place around 3 a.m. local time. She called on witnesses to come forward with information.

Interpol sent a bulletin alerting member countries to the theft, along with images of the stolen paintings.